1. Field of the Invention
The present invention relates to a metal corrosion inhibitor. More specifically, the invention concerns a corrosion inhibitor for use in a synthetic ester lubricating oil, particularly a synthetic ester turbo lubricating oil.
2. Description of Related Art
Lubricating oils are required to provide adequate lubrication over a wide range of operating conditions. In the case of turbo lubricating oils, synthetic basestocks normally are used to meet the wide operating range required of turbine engines in aircraft. As the operating conditions of the engines have become more severe (in part to achieve better fuel economy), the performance of lubricating oils has become more critical. The higher operating temperatures of the engine require greater thermal and oxidative stability of the basestock and additives. In addition, smaller clearances and tighter seals in the new engines result in considerably lower oil consumption. Hence, lubricating oil make-up rates for new engine designs are only a fraction of the make-up rates for older engine designs. As a result, turbo lubricating oils are now required which will perform at higher temperatures and for longer periods of time without degradation.
The use of amide-containing aromatics, such as anthranilamide, in lubricants for jet aircraft is known. For example, U.S. Pat. Nos. 3,585,137 and 3,850,824 (the disclosures of which are incorporated herein by reference) disclose the use of anthranilamide and its derivatives as a corrision inhibitor for turbo lubricating oil.
The use of substituted phenolics, such as propyl gallate, in lubricants also is known. For example, U.S. Pat. No. 3,790,478 (the disclosure of which is incorporated herein by reference) discloses the use of alkyl gallates (including propyl gallate) as a lead corrosion inhibitor in ester based lubricants.
While the addition of large amounts of the above-noted corrosion inhibitors may reduce the corrosion rate, the presence of such quantities in lubricating oils (particularly turbo lubricating oils) is undesirable because of their relatively low solubility (the solubility of anthranilamide and propyl gallate is only about 3000 and 2000 wppm, respectively, at 25.degree. C.). U.S. Navy specification MIL-L-23699C requires that a turbo lubricating oil be clear and free of any suspended material. In addition to this specification, the use of a turbo lubricating oil containing suspended matter would not be desirable, since the insoluble material could cause engine damage and, possibly, engine shutdown.
Accordingly, in view of the deficiencies of the prior art, it would be desirable to have available a corrosion inhibitor which will be effective for relatively long periods of time at high operating temperatures. It also would be desirable to provide a corrosion inhibitor for a turbo lubricating oil which will be compatible with the basestock and with other additives normally present. Further, it would be desirable to provide a corrosion inhibitor which is effective within the solubility limits of the basestock and at relatively low concentrations to reduce or eliminate the formation of insoluble material in a lubricating oil.